1. Field of the Invention
The present invention relates to an input switching circuit for a television tuner, which switches input between a television signal and an FM broadcasting signal.
2. Description of the Related Art
In a television tuner which allows reception of FM broadcasting signals, input to a tuning circuit is switched between a television signal and an FM broadcasting signal.
FIG. 3 shows the configuration of a conventional input switching circuit for such a television tuner. Referring to FIG. 3, between a first input terminal 41 to which a television signal is input and an output terminal 42, a high-pass filter 43, a trap circuit 44, and a first switching circuit 45 are provided, in that order. The high-pass filter passes the lowest frequency of the television signal (approximately 50 MHz) and higher, and the trap circuit 44 attenuates frequencies around 27 MHz which is used as the citizens"" band.
The first switching circuit 45 includes two switching diodes 45a and 45b connected in series between the first input terminal 41 and the output terminal 42, the anodes thereof being connected to each other, and a switching diode 45c, the cathode thereof being connected to the anodes of the switching diodes 45a and 45b and the anode thereof being grounded at high frequencies.
The node between the anodes of the switching diodes 45a and 45b and the cathode of the switching diode 45c is connected to a power supply terminal 46, and the cathodes of the switching diodes 45a and 45b and the anode of the switching diode 45c are connected to a switching terminal 47.
Meanwhile, between a second input terminal 48 to which an FM broadcasting signal is input and the output terminal 42, a high-pass filter 49 which passes the FM broadcasting signal, a second switching circuit 50, and a tuned circuit 51 which is tuned to the frequencies (88 to 108 MHz) of the FM broadcasting signal are provided, in that order. The second switching circuit 50 includes two switching diodes 50a and 50b connected in series between the second input terminal 48 and the output terminal 42, the anodes thereof being connected to each other, and a switching diode 50c, the cathode thereof being connected to the anodes of the switching diodes 50a and 50b and the anode thereof being grounded at high frequencies.
The node between the anodes of the switching diodes 50a and 50b and the cathode of the switching diode 50c is connected to the switching terminal 47, and the cathodes of the switching diodes 50a and 50b and the anode of the switching diode 50c are connected to the collector of a switching transistor 52.
The collector of the switching transistor 52 is connected to the power supply terminal 46, the base thereof is connected to the switching terminal 47, and the emitter thereof is grounded.
One end of the tuned circuit 51 is connected to the cathode of the switching diode 50b, and the other end thereof is grounded.
Furthermore, although not shown, an input tuning circuit which can be tuned to different frequencies of the television signal and the FM broadcasting signal is provided subsequent to the output terminal 42.
In the construction as above, when receiving a television signal, a low level switching voltage (e.g., 0 volts) is applied to the switching terminal 47. Then, in the first switching circuit 45, the switching diodes 45a and 45b are turned on and the switching diode 45c is turned off. Thus, the television signal input to the first input terminal 41 is passed through the first switching circuit 45 and output to the output terminal 42. Referring to FIG. 4, A indicates the transmission characteristics between the first input terminal 41 and the output terminal 42 at this time, in which frequencies around 27 MHz are attenuated and frequencies at and above approximately 50 MHz are transmitted.
At this time, the switching transistor 52 is turned off; thus, in the second switching circuit 50, the switching diodes 50a and 50b are turned off and the switching diode 50c is turned on. Thus, the FM broadcasting signal input to the second input terminal 48 is blocked by the second switching circuit 50, not being output to the output terminal 42.
Meanwhile, when receiving an FM broadcasting signal, a high level switching voltage (e.g., a voltage equivalent to the voltage applied to the power supply terminal 46) is applied to the switching terminal 47. Then, in the first switching circuit 45, the switching diodes 45a and 45b are turned off and the switching diode 45c is turned on. Thus, the television signal input to the first input terminal 41 is blocked, not being output to the output terminal 42.
At this time, the switching transistor 52 is turned on; thus, in the second switching circuit 50, the switching diodes 50a and 50b are turned on and the switching diode 50c is turned off. Thus, the FM broadcasting signal input to the second input terminal 48 is passed through the second switching circuit 50 and output to the output terminal 42.
As described above, in the second switching circuit 50, the switching diodes 50a and 50b are turned off and the switching diode 50c is turned on when receiving the television signal. However, at this time, the switching diodes 50a and 50b have a minute capacitance on the order of 0.5 pF across the terminals thereof, the terminals of the switching diodes 50c are not short-circuited completely, and the tuned circuit 51 is provided between the second switching circuit 50 and the output terminal 42 (i.e., externally to the tuned circuit 50); thus, the FM broadcasting signal input to the second input terminal 48 leaks from the second switching circuit 50, is tuned to by the tuned circuit 51, and appears at the output terminal 42.
Referring to FIG. 4, B indicates the transmission characteristics between the second input terminal 48 and the output terminal 42 at this time, in which a peak indicated by b is observed between 88 MHz and 108 MHz. Thus, interference due to the FM broadcasting signal occurs in receiving the television signal.
Accordingly, it is an object of the present invention to provide an input switching circuit for a television tuner, in which leakage of an FM broadcasting signal is reduced when receiving a television signal, preventing interference due to the FM broadcasting signal.
To this end, the present invention provides an input switching circuit for a television tuner. The television tuner includes a first input terminal, to which a television signal is input; a second input terminal, to which an FM broadcasting signal is input; an output terminal; a first switching circuit provided between the first input terminal and the output terminal, which, between the first input terminal and the output terminal, passes the television signal when receiving the television signal while blocking the television signal when receiving the FM broadcasting signal; and a second switching circuit provided between the second input terminal and the output terminal, which, between the second in put terminal and the output terminal, passes the FM broadcasting signal when receiving the FM broadcasting signal while blocking the FM broadcasting signal when receiving the television signal. The second switching circuit includes first two diodes connected in series with each other between the second input terminal and the output terminal, which are turned off when receiving the television signal and turned on when receiving the FM broadcasting signal; and a second diode connected between the node between the first two diodes and the ground, which is turned on when receiving the television signal and turned off when receiving the FM broadcasting signal. Furthermore, a parallel-tuned circuit which is tuned to the frequencies of the FM broadcasting signal is connected in parallel to the second diode. Thus, when receiving the television signal, the second switching diode inhibits the parallel-tuned circuit from tuning to the FM broadcasting signal, and therefore, the FM broadcasting signal input to the second input terminal is substantially blocked by the second switching circuit, the output level thereof at the output terminal being extremely low. Accordingly, no peak is observed in the FM broadcasting band, preventing interference due to the FM broadcasting signal.
Preferably, a series-tuned circuit including a first capacitor and an inductor is provided between the second input terminal and the second switching circuit, the series-tuned circuit being tuned to the frequencies of the FM broadcasting signal. Thus, when receiving the FM broadcasting signal, the series-tuned circuit, together with the parallel-tuned circuit, passes the FM broadcasting signal while attenuating signals at other frequencies.
More preferably, a second capacitor is connected between one end of the series-tuned circuit and the ground, the series-tuned circuit and the second capacitor constituting a low-pass filter at and above the frequencies of the FM broadcasting signal, the cutoff frequency of the low-pass filter being set between the lowest frequency of the VHF high band and the highest frequency of the FM broadcasting signal. Thus, when receiving the FM broadcasting signal, the television signal in the VHF band and above is attenuated, preventing interference due thereto.